Is it permissible to sever ties with relatives (maternal aunts) if their actions involve insult and slander, or dealing only out of self-interest, or fabricating lies and engaging in bad behavior? And would the questioner be considered a severer of kinship ties in this case?

The default is the obligation to maintain ties of kinship, and their harming of your mother does not nullify this obligation. Rather, it is obligatory to maintain ties with them and enjoin them to good and forbid them from evil. An exception to this is if it is highly probable that boycotting them will lead them to cease their mischief and harm; in such a case, boycotting is permissible to achieve this benefit, and it is not considered a severance of ties. Rather, it is part of fulfilling the complete kinship ties and support required.

Similarly, if your boycotting them or one of them is to prevent harm that would befall you, there is no blame upon you for boycotting, because a beautiful avoidance may be better than harmful interaction.

The Sharia has not specified a particular method for maintaining ties of kinship; rather, it refers to custom and varies according to people's circumstances. Therefore, it is sufficient to inquire about them using modern means of communication, and there are varying degrees of maintaining ties, the lowest of which is refraining from abandoning them and exchanging greetings.